Electrostatic interactions (EI) in denatured state of proteins are an important factor regulating folding/unfolding equilibria in these macromolecules. Therefore, a physically adequate model for description of EI in denatured proteins is highly desirable. For this purpose, unfolded polypeptide chains with excluded volume and charge-charge interactions taken into consideration were studied by means of Monte Carlo simulations. Charge-charge interactions were calculated using the Coulomb law both with constant, epsilon(s), and distance dependent, epsilon(r), dielectric permittivity. Average dimensions, in terms of radius of gyration, R-g, for chains of different lengths N-r, were obtained within a wide temperature range and for various distributions of positive and negative charges. The results suggest that unfolded proteins can adopt two distinct states, compact and expanded, depending on temperature and charge composition. The compact state is characterized by R-g close to that of native proteins, while the expanded state has R-g corresponding to a flexible homopolymer with excluded volume interactions only. A simple procedure for evaluation of R-g of unfolded proteins at different pH is proposed and the predicted R-g values are compared to the experimental data for fully unfolded states for several soluble denatured proteins. (C) 2003 American Institute of Physics.